1. Technical Field
The invention relates to plasma processing and plasma processing equipment. More particularly, the invention relates to an apparatus for generating a plasma torch that is focused directly into a process chamber, thereby improving the process, for example the etch profile control of a substrate in the chamber.
2. Description of the Prior Art
Plasma processing is an essential tool of the semiconductor manufacturing industry. Such processing uses electromagnetic radiation to dissociate the molecules of a reactive species and thereby produce a plasma that is directed to the surface of a workpiece, such as a semiconductor wafer, in a process environment, e.g. a vacuum chamber.
A plasma may be produced directly above the surface of the wafer within the process environment, or the plasma may be remotely generated in an applicator, and then conducted to the surface of the wafer. FIG. 1 is a schematic diagram showing a plasma processing environment 10 according to the prior art in which the plasma is remotely generated in an applicator. The plasma processing environment includes a process chamber 12 and an applicator 14 for remotely generating a plasma. The typical plasma applicator comprises a tube, open at at least one end, through which a reactive gas is pumped.
The reactive gas is supplied from a source 16 and may comprise such well known process gases as CF.sub.4, O.sub.2, N.sub.2, Cl.sub.2, NF.sub.3, and methanol. A waveguide 18 is provided to direct a source of electromagnetic energy 20 to the tube to ignite and maintain a plasma therein. The source of electromagnetic energy may be either an RF signal (typically having a frequency of 13.56 MHz) or it may be (and preferably is) a microwave signal (typically having a frequency of 2.45 GHz).
The plasma thus generated is directed out of the open end of the tube, through a transport conduit 22 toward the process chamber 12, in which a gas distribution plate 26 may be used to spread neutrals and ions for even distribution therein. The neutrals and ions are directed towards a workpiece, typically a semiconductor wafer 28. A wafer support 30 is provided that may also be used as a cathode, while the walls of the process chamber may be used as an anode. After the reactive gas is spent, for example after etching the semiconductor wafer, the gas is exhausted from the process chamber through an exhaust port 32.
One problem encountered in prior art applicators of a remote plasma source is that of species recombination. Reactive species, such as excited species or ions, that are not quickly delivered from the applicator and delivery tube to the process chamber can be changed either from ions to neutrals or from an excited state to a ground state. The recombination could be due to a collision between the ions, electrons, or neutrals, or a collision with the wall of the delivery tube.
Once the plasma has been ignited, it must be sustained. However, in a plasma etch chamber according to the prior art, the composition of ions is not controllable because different reactive species have different levels of ionization energy. The ionization rates of different gases in a gas mixture are therefore determined by the minimum ionization energy of the gases. For example, the minimum ionization energy for Cl.sub.2 is approximately 11.5 eV, while that for Ar is 15.8 eV. Thus, in a Cl.sub.2 and Ar mixture, the density of Cl.sup.+ and Cl.sub.2.sup.+ ions is much higher than the density of Ar.sup.+ ions. Changes in RF power, chamber pressure, or flow ratio do not effectively change the composition of ions.
It would therefore be advantageous to provide an apparatus and method for providing a secondary or activating plasma source to a process chamber that minimizes losses of ions and reactive species due to recombination. It would be a further advantage if an apparatus and process were available that could control the composition of ions in the process chamber, to thereby improve the etch process and control the etch profile of a substrate being process within the process chamber.